1. Field of the Invention
The invention relates to fluorescent lamps, and is directed more particularly to a cold cathode subminiature fluorescent lamp with an electrode configuration enabling operation at high current densities and concomitant high illuminance levels.
2. Description of the Prior Art
Subminiature fluorescent lamps, i.e., those having a diameter of 7 mm or less, with high lumen per watt efficiency, are employed where low power consumption and/or low thermal loading is essential. In many instances, such small diameter lamps replace several incandescent bulbs, and high surface brightness and total luminance levels are required.
Brightness in such low-pressure discharge lamps is directly proportional to lamp current applied. However, high currents, in conjunction with the small diameters of the lamps and the compact electrodes employed, often result in excessive current densities at the cathode. Early failures of subminiature lamps operated at high current densities have been observed and causally linked to the tendency of the discharge to dwell, or "root", on the lead wires used to supply electrical power to the electrodes.
Referring to FIGS. 1 and 2, at low lamp currents, the discharge concentrates on portions of the cold cathode lamp electrode 10, and the affected area glows. As the current in the lamp is increased, the discharge envelops the available surface area of the electrode 10 and moves down the lead wires 12, 14. The discharge roots on the lead wires 12, 14, a phenomenon which is readily viewable as a bright glow on the electrode and lead wires. The lamp envelope 16 typically is a soda lime glass and, when the lead wires 12, 14 are enveloped by the discharge, a reaction occurs at the lead wire to glass envelope interface 18 which yields free sodium. In due course, the glass to lead wire seal at the interface 18 is compromised by the reaction and the lamp vents to the atmosphere and fails. Such a failure often occurs in as little as 100 hours of operation in lamps having a rated life of 10,000 hours.
Accordingly, there exists a need for a cold cathode subminiature fluorescent lamp which is not subject to premature failure caused by discharge rooting on the lead wires and consequent depletion and failure of the lead wire to glass envelope seal.